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| <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b> Loops</b></font> |
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| <p>So far we have introduced a couple of EBNF operators that deal with looping. |
| We have the <tt>+</tt> positive operator, which matches the preceding symbol |
| one (1) or more times, as well as the Kleene star <tt>*</tt> which matches the |
| preceding symbol zero (0) or more times.</p> |
| <p>Taking this further, we may want to have a generalized loop operator. To some |
| this may seem to be a case of overkill. Yet there are grammars that are impractical |
| and cumbersome, if not impossible, for the basic EBNF iteration syntax to specify. |
| Examples:</p> |
| <blockquote> |
| <p><img src="theme/bullet.gif" width="12" height="12"> A file name may have |
| a maximum of 255 characters only.<br> |
| <img src="theme/bullet.gif" width="12" height="12"> A specific bitmap file |
| format has exactly 4096 RGB color information. <br> |
| <img src="theme/bullet.gif" width="12" height="12"> A 32 bit binary string |
| (1..32 1s or 0s).</p> |
| </blockquote> |
| <p>Other than the Kleene star <tt>*</tt>, the Positive closure <tt>+</tt>, and |
| the optional <tt>!</tt>, a more flexible mechanism for looping is provided for |
| by the framework. <br> |
| </p> |
| <table width="80%" border="0" align="center"> |
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| <td colspan="2" class="table_title">Loop Constructs</td> |
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| <td class="table_cells" width="26%"><b>repeat_p (n) [p]</b></td> |
| <td class="table_cells" width="74%">Repeat <b>p</b> exactly <b>n</b> times</td> |
| </tr> |
| <tr> |
| <td class="table_cells" width="26%"><b>repeat_p (n1, n2) [p]</b></td> |
| <td class="table_cells" width="74%">Repeat <b>p</b> at least <b>n1</b> times |
| and at most <b>n2</b> times</td> |
| </tr> |
| <tr> |
| <td class="table_cells" width="26%"><b>repeat_p (n, more) [p] </b></td> |
| <td class="table_cells" width="74%">Repeat <b>p</b> at least <b>n</b> times, |
| continuing until <b>p</b> fails or the input is consumed</td> |
| </tr> |
| </table> |
| <p>Using the <tt>repeat_p</tt> parser, we can now write our examples above:</p> |
| <p>A file name with a maximum of 255 characters:<br> |
| </p> |
| <pre> <span class=identifier>valid_fname_chars </span><span class=special>= </span><span class=comment>/*..*/</span><span class=special>; |
| </span><span class=identifier>filename </span><span class=special>= </span><span class=identifier>repeat_p</span><span class=special>(</span><span class=number>1</span><span class=special>, </span><span class=number>255</span><span class=special>)[</span><span class=identifier>valid_fname_chars</span><span class=special>];</span></pre> |
| <p>A specific bitmap file format which has exactly 4096 RGB color information:<span class=special><br> |
| </span></p> |
| <pre> <span class=identifier>uint_parser</span><span class=special><</span><span class=keyword>unsigned</span><span class=special>, </span><span class=number>16</span><span class=special>, </span><span class=number>6</span><span class=special>, </span><span class=number>6</span><span class=special>> </span><span class=identifier>rgb_p</span><span class=special>; |
| </span><span class=identifier>bitmap </span><span class=special>= </span><span class=identifier>repeat_p</span><span class=special>(</span><span class=number>4096</span><span class=special>)[</span><span class=identifier>rgb_p</span><span class=special>];</span></pre> |
| <p>As for the 32 bit binary string (1..32 1s or 0s), of course we could have easily |
| used the <tt>bin_p</tt> numeric parser instead. For the sake of demonstration |
| however:<span class=special><br> |
| </span></p> |
| <pre> <span class=identifier>bin</span><span class=number>32</span> <span class=special>= </span><span class=identifier>lexeme_d</span><span class=special>[</span><span class=identifier>repeat_p</span><span class=special>(</span>1, <span class=number>32</span><span class=special>)[</span><span class=identifier>ch_p</span><span class=special>(</span><span class=literal>'1'</span><span class=special>) </span><span class=special>| </span><span class=literal>'0'</span><span class=special>]];</span></pre> |
| <table width="80%" border="0" align="center"> |
| <tr> |
| <td class="note_box"><img src="theme/note.gif" width="16" height="16"> Loop |
| parsers are run-time <a href="parametric_parsers.html">parametric</a>.</td> |
| </tr> |
| </table> |
| <p>The Loop parsers can be dynamic. Consider the parsing of a binary file of Pascal-style |
| length prefixed string, where the first byte determines the length of the incoming |
| string. Here's a sample input: |
| <blockquote> |
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| <td class="white_bkd" width=8%"> |
| <div align="center">11</div> |
| </td> |
| <td class="white_bkd" width="8%"> |
| <div align="center">h</div> |
| </td> |
| <td class="white_bkd" width="8%"> |
| <div align="center">e</div> |
| </td> |
| <td class="white_bkd" width="8%"> |
| <div align="center">l</div> |
| </td> |
| <td class="white_bkd" width="8%"> |
| <div align="center">l</div> |
| </td> |
| <td class="white_bkd" width="8%"> |
| <div align="center">o</div> |
| </td> |
| <td class="white_bkd" width="8%"> |
| <div align="center"> _</div> |
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| <td class="white_bkd" width="8%"> |
| <div align="center">w</div> |
| </td> |
| <td class="white_bkd" width="8%"> |
| <div align="center">o</div> |
| </td> |
| <td class="white_bkd" width="8%"> |
| <div align="center">r</div> |
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| <td class="white_bkd" width="8%"> |
| <div align="center">l</div> |
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| <td class="white_bkd" width="8%"> |
| <div align="center">d</div> |
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| |
| </blockquote> |
| <p>This trivial example cannot be practically defined in traditional EBNF. Although |
| some EBNF syntax allow more powerful repetition constructs other than the Kleene |
| star, we are still limited to parsing fixed strings. The nature of EBNF forces |
| the repetition factor to be a constant. On the other hand, Spirit allows the |
| repetition factor to be variable at run time. We could write a grammar that |
| accepts the input string above:</p> |
| <pre><span class=identifier> </span><span class=keyword>int </span><span class=identifier>c</span><span class=special>; |
| </span><span class=identifier>r </span><span class=special>= </span><span class=identifier>anychar_p</span><span class=special>[</span><span class=identifier>assign_a</span><span class=special>(</span><span class=identifier>c</span><span class=special>)] </span><span class=special>>> </span><span class=identifier>repeat_p</span><span class=special>(</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>ref</span><span class=special>(</span><span class=identifier>c</span><span class=special>))[</span><span class=identifier>anychar_p</span><span class=special>];</span></pre> |
| <p>The expression</p> |
| <pre> <span class=identifier>anychar_p</span><span class=special>[</span><span class=identifier>assign_a</span><span class=special>(</span><span class=identifier>c</span><span class=special>)]</span></pre> |
| <p>extracts the first character from the input and puts it in <tt>c</tt>. What |
| is interesting is that in addition to constants, we can also use variables as |
| parameters to <tt>repeat_p</tt>, as demonstrated in </p> |
| <pre> <span class=identifier>repeat_p</span><span class=special>(</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>ref</span><span class=special>(</span><span class=identifier>c</span><span class=special>)</span><span class=special>)</span><span class=special>[</span><span class=identifier>anychar_p</span><span class=special>]</span></pre> |
| <p>Notice that <tt>boost::ref</tt> is used to reference the integer <tt>c</tt>. |
| This usage of <tt>repeat_p</tt> makes the parser defer the evaluation of the |
| repetition factor until it is actually needed. Continuing our example, since |
| the value 11 is already extracted from the input, <tt>repeat_p</tt> is is now |
| expected to loop exactly 11 times.</p> |
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| <p class="copyright">Copyright © 1998-2003 Joel de Guzman<br> |
| <br> |
| <font size="2">Use, modification and distribution is subject to the Boost Software |
| License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at |
| http://www.boost.org/LICENSE_1_0.txt) </font> </p> |
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